Uruguayan scientists have unearthed the fossil of a prehistoric rodent they say weighed over 1,000 kg and could have been as much as three metres long, making it the largest rodent ever found.

The scientists base their findings, published Wednesday, on the discovery of a skull measuring 53 cm in length that they say belongs to an entirely new species they have named Josephoartigasia monesi.

They said the creature, which is now extinct, lived in South America four million years ago alongside saber-tooth cats and ground sloths.

Andres Rinderknecht and Ernesto Blanco of the National Museum of Natural History in Montevideo reported their findings in Britain's Proceedings of the Royal Society B: Biological Sciences.

The largest living rodent today is the guinea pig-like capybara, also found in South America. It can grow to about 60 kg.

"We can give an educated guess that the rodent would have been 3 metres long — assuming that it was similar to a Capybara (the largest rodent alive today) and taking into account that large mammals generally have relatively smaller heads," Blanco said in a statement. "Its tail probably was closer to the one of capybara or guinea pig (very short) and not like a rat."

While the scientists speculate the rodent was as large as a modern-day bull, its jaws were not particularly powerful. They estimate the rodent would have survived on soft plants and fruit — not the usual diet of a rodent.